Gibberellins are a large family of closely related tetracyclic triterpenoid compounds first discovered as metabolites of an Ascomycete, Gibberella fujikuroi (perfect state of Fusarium moniliforme), which causes the bakanae disease of rice seedlings. The disease is typified by excessive stem and leaf elongation. Infected seedlings became abnormally tall and spindly and usually fall over. The culture filtrate of Gibberella fujikuroi produced a similar growth-promoting effect. From the filtrate a crystalline active product, later shown to be a mixture of gibberellins, was isolated. Gibberellins assumed a wider significance when it was discovered that gibberellins (many of which have not been detected in G. fujikuroi) are endogenous plant growth hormones.
One hundred and twenty-one gibberellins have been described (http://www.plant-hormones.bbsrc.ac.uk/gibberellin_information2.htm) and named gibberellin A.sub.1, A.sub.2, A.sub.3, A.sub.4, A.sub.5, . . . , approximately in the order they were discovered. Gibberellin A.sub.3 (GA.sub.3), GA.sub.4, and GA.sub.7 pertinent to this invention and their chemical structures are shown below. ##STR1##
Since gibberellins are endogenous plant hormones, their concentrations in plant tissues are quite low and are tightly regulated. Immature seeds, the best source of plant derived gibberellins, contain 10-100 .mu.g per gram wet weight. Gibberella fujikuroi is by far the most abundant source of the gibberellins, and GA.sub.3 is usually the major gibberellin component. Fermentation titers of GA.sub.3 at 1,000 mg/liter have been achieved in commercial production, although the actual titer is purported to be a few times higher, and titers of 2,000 mg/liter have been claimed (Bruckner, B. & Blechschmidt, D.: The Gibberellin Fermentation, Critical Reviews in Biotechnology, 11(2), 163-192 (1991)).
Different gibberellins stimulate the growth of different parts of plants and are effective during different periods of a plant's growth cycle. Gibberellin A.sub.3 primarily stimulates the growth of stems and leaves, while GA.sub.4 and GA.sub.7 primarily stimulate flowering and cause fruit cells to elongate. Mixtures of GA.sub.4 and GA.sub.7 (GA.sub.4+7) have been used successfully by growers of apples, pears and grapes to produce larger fruits and to achieve earlier harvests. The commercial production of GA.sub.4 and GA.sub.7, however, has not been quite as successful as that of GA.sub.3, since GA.sub.4 and GA.sub.7 generally are minor metabolites of Gibberella fujikuroi. Titers of GA.sub.4+7 at .about.650 mg/liter with a GA.sub.4 /GA.sub.7 ratio of 1:5.about.6 have been reported previously (ICI Patent, EP 0112629B, 29.04.87). Furthermore, the separation of GA.sub.4 and GA.sub.7 from a mixture containing both is difficult and has not been economically feasible; thus, GA.sub.4 and GA.sub.7 are available commercially only as mixtures with GA.sub.7 as the predominant component. As fine chemicals, the costs of GA.sub.4 and GA.sub.7 are approximately 300 times that of GA.sub.3.
Many other applications of GA.sub.4+7 have been demonstrated and documented (Commercial uses of gibberellins, in The Biochemistry and Physiology of Gibberellins, Vol. 2, Crozier A., Ed., Praeger, New York, 1983). For example, the use of GA.sub.4+7 for russet control of apples have demonstrated that the GA.sub.3 in the GA.sub.4+7 mixture caused reduction in flower bud formation and that GA.sub.4 is most effective in russet control. GA.sub.4 was found to be superior to GA.sub.3 and GA.sub.4+7 for promoting fruit set of many commercially grown apples. GA.sub.4+7 was found to induce flowering in seed plants including many coniferous species (Pharis, R. P. & King, R. W.; Gibberellins & reproductive development in seed plants, Rev. Plant Physiol., 36, 517, 1985), as well as promote seed cone production (Ho, R. H., Gibberellin A.sub.4+7 enhances seed cone production in field-grown black spruce, Can. J. For. Res., 18, 139, 1988), opening up the possibility of using GA.sub.4+7 in forestry management.
A number of laboratories have searched for high producing strains of GA.sub.4 and GA.sub.7 that produce little or no GA.sub.3. Sphaceloma manihoticola, a fungus that causes the super-elongation disease of cassava, produced GA.sub.4 as the major gibberellin component (Graebbe, J. E., and Rademacher, W. B.; EP 0024951 B1). It also produced a number of other gibberellins, but no GA.sub.3 and GA.sub.7. The fermentation titer of GA.sub.4, however, was only .about.7 mg/liter and not of commercial importance. The laboratory of E. Cerda-Omedo reported a number of strains of Gibberella fujikuroi that produced GA.sub.7 at the expense of GA.sub.3 (Gibberellin Biosynthesis in gib mutants of Gibberella fujikuroi, J. Biol. Chem., 270:25, 14970-14974, 1995). The GA.sub.7 titer, however, is only in the range of 60-80 mg/liter.